This invention is directed to the problem of disposing of ultra-toxic industrial wastewater streams containing high levels of toxins. Such an ultra-toxic stream is obtained as "feed" for this process, after a plant-wastewater stream containing "grey" and "black" water has been preliminarily treated. After such treatment, as will be explained herebelow, the "feed" obtained contains toxins such as ammonia, phenols, chlorocarbons, aromatic and aliphatic hydrocarbons, and the like, which could not have been degraded by the preliminary treatment.
This process converts the toxins to products such as carbon dioxide and water by biochemical oxidation, instead of separating the toxins with some type of adsorbent or separation medium (together referred to as "media"). The investment and cost of using such media is high, and the cost of disposing of the media after it has sorbed (whether adsorbed, absorbed, or otherwise) the toxins, is typically just as high, if not higher. The object was to develop a more cost-effective solely biological process than any currently used, or deemed suitable for use, preferably an aerobic process which would generate only a minimal amount of sludge to be disposed of, most preferably none.
This invention uses an aerobic process which stems from the discovery that available, acclimated microorganisms or bacteria, can quickly ingest and degrade toxins provided the toxins are fed to them at a sufficiently low concentration. Microorganisms or bacteria used in this and other processes for a similar purpose, are mainly unicellular organisms having a nominal diameter less than 2 .mu.m, and more typically, about 1 .mu.m or even smaller. These organisms will be referred to hereinafter as "cells" for convenience and brevity.
From a study of the prior art, the most relevant of which will be referred to herebelow, it appeared that a very large number of cells, adsorbed on a very high surface area adsorbent is necessary to degrade relatively low levels of toxins. The references appear not to have realized that relatively few cells can effectively dispose of a toxin at the same rate (measured as toxin removal rate, mg/min) as a much larger number of cells, if a large proportion of the latter cells are not able to feed on the toxin because they are engulfed with too much of the toxin; and, in contrast, the toxin is delivered to the former at a "regulated" or "metered", ingestible and degradable rate.
Stated differently, specific cells can only degrade as much of the toxin for which they are specifically acclimated, and no more. A cell will degrade even a relatively concentrated toxin if the amount fed to it is minimal. A cell will degrade a relatively large amount of the toxin if present in a dilute solution. It serves no useful purpose to serve a cell more toxin than it can ingest, and the cell will ignore the too-large amount of toxin, or the excess, until the excess is sufficiently large to affect the cell's well-being adversely. Thus, the logical deduction is: the more cells the better.
As a result, prior art solutions have relied on providing a biosupport having as large a surface area as possible, based on the fact that, in a process operating at equilibrium, cells cover every available square unit of surface. Therefore, the more surface area, the more the cells. Thus, the prior art has provided a host of processes (some are referred to in greater detail herebelow) using various adsorbents, specific combinations of adsorbents, and an adsorbent entrapped in a macroporous open-cell foam. (It will be recognized that the adjective "open-cell" simply describes the fact that the cells in the foam are in open communication with each other, and has nothing to do with a "cell" which biologically ingests and degrades toxin.)
However, for reasons which are not fully understood, simply having a very large surface area is not sufficient to expose all the cells in such a manner that each is exposed to an optimum level of toxin which the cell regards as a nutrient. For example, even when a very large number of cells are lodged on, and within, a high surface area activated carbon which is encapsulated in a macroporous foam (see Lupton et al, below), the removal rate of even a relatively low concentration of toxin is not commensurate with the total number of cells (as a function of surface area) available for the task.
Accordingly, we reasoned that a large number of cells, when fed with a toxin in high concentration in a feed, were unable to "see" their proffered nutrient in a concentration low enough to be ingested and degraded with dispatch. Particularly since a large surface area can realistically be provided only by porous supports, we reasoned that a large number of cells within the pores of the support were not, for one reason or the other, being proffered the nutrient at a concentration and in an amount they preferred, therefore could not ingest the toxin. Further, those cells at or near the surface, which were being presented with the "just right" concentration, were also presented with a much larger amount than they could degrade, therefore the cells ignored the remainder. The solution to the problem appeared to require that a very large number of cells be present, and that they be presented their nutrient (toxin) not only in an attractive concentration, but also in a usable amount. Since there appeared to be no problem lodging a very large number of cells, this being a function of surface area, it appeared that the real problem would be to find a way to "meter" the toxin to the cells in no higher an amount, and in no higher a concentration than they can ingest and degrade.
It was disclosed in U.S. Pat. No. 4,581,338 to Robertson et al, that pore diameters of from 1-25 .mu.m are needed to accommodate cells which are about 1 .mu.m in diameter, and that either a porous gel, usually an alginate gel, or a porous high-silica pellet, or one of a mixture of silica and alumina, provided the requisite pore size. But they stated that the use of gels is not without problems (bottom of col 1), and that the average pore diameter in the silica pellet was too small relative to the 1-25 .mu.m diameter "needed to accommodate microbial cells"; therefore, concluded that "the economic attractiveness of such a support in commercial processes is greatly reduced" (see col 2, lines 39-48).
Despite the proscription relating to the use of small-pore supports, we were able to find in this teaching, the basis of a solution to our problem. We found that microporous open-cell supports, having particular cell geometries which are specific to those supports, provide "windows" or "openings" which serve as orifices to meter just the right amount of nutrient to the cells lodged on the available total surface of the support, both the exterior surface as well as the interior surfaces of the support's cells, all and each of which is covered with a colony or colonies of cells (microorganisms).
Some microporous supports, whether a solid synthetic resin or a naturally occurring material, simply do not have the right chemical compatibility for most cells. In this specific respect, namely the chemical composition of the foam, note that Lupton et al (U.S. Pat. No. 4,983,299) state that the particular composition of the very large pore foam they used, was a relatively unimportant aspect of their invention, therefore they simply chose polyurethane for ease and convenience. We found that not only did the chemical composition of the foam make a difference, but also its physical structure particularly with respect to the geometry of the pores.
Further we found that, despite having more than adequate surface area, some microporous open-cell resins (whether foams or not) which are known to have extreme chemical inertness, such as polytetrafluoroethylene (PTFE) do not appear to have requisite chemical compatibility demanded by cells which feed on highly recalcitrant toxins. Other resins appear not to have a requisite microorganismcompatible geometry with that demanded by these cells, for the metering task. Some resins have neither the desired chemical microorganism-compatibility nor the requisite geometry, for example, PTFE. Quite unexpectedly, various microporous clays (e.g. calcined diatomite, commercially available as Celite.RTM. from Johns Manville Corp), and activated carbon, both of which have relatively high surface areas appear not to possess either the desired chemical compatibility or the requisite geometry.
We believed that the discovery of how to meter an ingestible amount of toxin to the cells with micropores of requisite geometry, could be effectively implemented in a biochemical oxidation process, though there is no reasonably practical method now available to determine the precise geometry which will satisfy particular cells. We have now determined that the essential feature of such a process is a fixed or immobilized packed bed of particular (chemical) types of microporous synthetic resinous packing having micropores with preferred "just right" geometries, in essentially all of which micropores especially acclimated colonies of cells are lodged without being held in suspension. When fed with enough nutrient (pollutant or toxin) to maintain the health of the securely ensconced colonies, these acclimated cells generate only so much growth as will lead to a net production of less than 10% (based on the chemical oxygen demand, "COD", of the toxin degraded) of the cell population. This is also noted in Lupton et al's fixed bed process, specifically to degrade phenols.
This process is not concerned with the bioremediation of domestic or municipal wastewater streams such as are typically treated in a municipal wastewater treatment plant, though the presence of a minor proportion of such domestic or municipal waste might either be adventitiously present, or may be deliberately included to provide additional nutrients for the especially acclimated cells used in this process.
More specifically, though it will be obvious that lower concentrations of even a highly recalcitrant toxin, less than about 40 ppm, can be removed from a feed with a conventional activated sludge process, or the known PACT process, the microporous packed bed of this invention is used to remove much higher concentrations of the highly recalcitrant toxin, in a process for the aerobic degradation of wastewater containing:
(a) from about 40-1000 mg/L (=40-1000 ppm) of ammonia by an inoculum (or culture) of Nitrosomonas and Nitrobacter; PA1 (b) from 40-1000 mg/L of phenolic compounds, particularly phenol and resorcinol, PA1 (c) from 40-1000 mg/L of aliphatic and cycloaliphatic hydrocarbons, such as C.sub.4 -C.sub.12 alkanes, e.g. butane, pentane; C.sub.5 -C.sub.8 cycloalkanes, e.g. cyclohexane and cycloheptane; and C.sub.4 -C.sub.12 alkenes, e.g. butene, hexene; C.sub.7 -C.sub.28 cycloolefins, e.g. norbornene, dicyclopentadiene; PA1 (d) from 40-1000 mg/L of aromatic hydrocarbons, such as benzene, toluene, naphthalene, anthracene, etc.; PA1 (f) from 40-1000 mg/L of rubber chemicals such as mercaptothiazoles (MBT), MBT disulfide, MBT sulfenamide, etc.; PA1 (g) from 40-1000 mg/L of aliphatic and aromatic amines, for example, diethylamine, cyclohexylamine, aniline and aralkyamines such as alkylated diphenylamine; and, PA1 (h) from 40-1000 mg/L of a halogenated organic compound, for example, a haloalkane such as 1,2-dichloroethane (EDC), perchloroethylene (PERC), a haloalkylene oxide such as epichlorohydrin, or a halogenated aromatic compound such as a chloro- bromo- or iodobenzene.
Except for the inoculum for the biodegradation of ammonia which is obtained from a laboratory culture, the inocula for the other wastewater streams are either obtained from wastewater contaminated with the compound(s) to be treated, or from soil around a pond of wastewater containing the compound(s) to be treated.
By the prefix "halo" we refer to compounds containing at least one chlorine, bromine or iodine atom which is covalently bonded in the compound's structure. Each inoculum used is obtained from the sources stated, those from soil contiguous to the contaminated wastewater being obtained because the wastewater itself is generally too toxic to support life of most known cells.
Each of the aforementioned organic compounds, even when present in a relatively low concentration, in the range &gt;40 ppm but &lt;100 .mu.g/L, are known to be poisons for most common bacteria, particularly for those such as are generally used in a conventional municipal facility utilizing an activated sludge process. In the relatively low concentration range of from 40-100 mg/L, such organic compounds are essentially non-biodegradable by common bacteria.
Conventional domestic activated sludge such as is available from a typical municipal wastewater treatment plant is wholly unsuitable for use in our fixed packed bed process in which viable, aerobic cells are immobilized in the pores of a microporous synthetic solid resinous packing, familiarly referred to herein as "porous plastic". For this reason, an inoculum for use in our fixed packed bed is either especially cultured, or is isolated from a suitable source containing the toxins to be treated, and the inoculum is especially acclimated to ingest and biodegrade those toxins, such acclimation being accomplished by techniques which are well known in the art. The inoculum is thus specifically adapted to biodegrade the feed from which it is known to derive all or part of the nutritional requirements of the cells; therefore the inoculum is said to be adapted for a particular biodegradation duty.
As a result of the ultra-toxic nature of such industrial wastewater streams which are the "feed" to our process, unlike a municipal wastewater or sewage-containing stream, such ultra-toxic streams are generally treated with activated carbon, steam stripping and other comparably effective physical treatments. All these approaches, in principle, simply result in a physical exchange of the matrix which is contaminated, and hence are not environmentally friendly. In the PACT process the toxins are removed from the feed but remain on the adsorbent used. The adsorbent must then be disposed of. If the adsorbent is calcined, the toxins are released to the atmosphere.
Where a typical industrial wastewater stream, after primary treatment contains highly recalcitrant organics, or after secondary treatment, contains high ammonia-nitrogen concentration (greater than 40 ppm) the effluent is treated by additional means. Primary treatment comprises pH adjustment and solids settling; secondary treatment comprises biochemical oxidation using suspended cells (activated sludge treatment). A chlorocarbon stream containing &gt;40 ppm of chlorocarbons cannot be biochemically treated conventionally. A typical feed containing &gt;40 ppm ammonia-nitrogen will upon neutralization be ignored in the conventional secondary treatment because the ammonia-nitrogen containing salts are not toxic.
The typical industrial wastewater stream just referred to hereinabove, when fed to a conventional activated sludge treatment, has a relatively low BOD (biological oxygen demand), the ratio of BOD to COD being very low, typically less than 0.2 (&lt;0.2). In contrast, a typical municipal wastewater stream has a BOD to COD ratio of greater than 0.6 (&gt;0.6) because the stream contains mainly "grey" and "black" water. By "grey" water we refer to wash water from a sink, shower stall or bath, kitchen water including water from washing food-soiled dishes, laundry water and the like, all with detergents and/or soaps. By "black" water we refer mainly to water containing sewage from toilets, and effluent streams from meat packing plants and the like.
Since the cells are acclimated to degrade "feed" in the process we shall describe, the COD is essentially equal to the BOD, because what normally is non-biodegradable matter (and is therefore normally measured only as COD) is now consumed by the acclimated cells of the novel process as nutrients (carbon source). A typical chlorocarbon wastewater "feed" contains chlorocarbons in the range from 40-1000 mg/L, more typically from 65-600 mg/L. A typical ammonia-rich "feed" contains a number of nitrogenous compounds which are typically reported as "ammonia-nitrogen" which may range from 40-1000 mg/L, more typically 75-500 mg/L.
Ammonia is converted to nitrite by the Nitrosomonas cells, and the Nitrobacter cells convert the nitrite to nitrate. In addition to ammonia, the cells need to be supplied with a source of inorganic carbon and oxygen to effect nitrification. Such nitrification is accompanied with production of acid and the alkalinity of the influent wastewater is neutralized causing a drop in pH. A convenient means for supplying a combination of all three needs of the process is to maintain the pH of the bed by the addition of sodium carbonate or lime or caustic, or a combination thereof.
In such streams, aerobic bacteria, if adequately "protected" are able to withstand exposure to the ultratoxic compound(s), because the bacteria are nourished by the non-toxic easily biodegradable organic solids, while managing successfully to ignore the presence of the ultratoxic compounds. Because such wastewater streams are amenable to purification, albeit with primary, secondary and tertiary treatments, such wastewater streams are referred to herein as "relatively recalcitrant" wastewater streams. The process of this invention deals with even more recalcitrant wastewater streams, hence referred to as "highly recalcitrant" streams.
Such "protection" in the prior art, is provided by an adsorbent chosen to adsorb the toxin, as for example, in the well known PACT process in which either activated carbon, or a mixture thereof with Fuller's earth, is mixed into the wastewater, as will be described in greater detail herebelow.
It will immediately be evident that the higher the ratio of ultra-toxic toxin to non-toxic organic compounds in conventional wastewater, the less likely it is that cells will survive, and even less likely that they will replicate. When the BOD in such a wastewater stream is less than one-half the COD, even processes in which the stream is contacted with an adsorbent, fail to provide a bioreactor in which colonies of cells thrive at equilibrium conditions, and replicate regularly to maintain such conditions. The process of this invention is directed to either the secondary, and more typically, the tertiary treatment of highly recalcitrant streams in which the BOD is less than one-half the COD.
An example of a PACT process is the treatment, in a bioreactor, of a wastewater stream having a pH in the range from 4 to 11, and a total suspended solids (TSS) content of between 10 and 50,000 ppm (parts per million parts of feed) such as the process provided in U.S. Pat. No. 4,069,148 to Hutton et al.
This wastewater stream was specifically required to be mixed with a finely divided adsorbent having a surface area of at least 100 m.sup.2 (square meters) per gram, the adsorbent being a mixture of from 5-50,000 ppm of activated carbon and from 25-2500 ppm adsorptive Fuller's earth, to form a suspension of the adsorbent in the wastewater. The suspension was then aerated and the adsorbent was then removed from the bioreactor.
As stated in the '148 patent "Not only does the presence of carbon or fuller's earth cause a segregation of poisonous impurities, but it also causes greater percentage of BOD removal, and it does so in a much shorter time." (see col 6, lines 10-13). Clearly, the poisonous impurities are not subjected to biochemical degradation but were simply adsorbed by the adsorbent mixed into the wastewater, rather than being ingested and degraded by the cells. Moreover, there is no indication in the '148 reference as to what the ratio of BOD to COD in a waste-water treatable by their process may have been; nor is the range of BOD to the `total organic carbon` (TOC) specified. Most important, the only identification of the "poisonous impurities" treatable are heavy metal compounds, specifically lead, chromium and cobalt, which of course, are not biodegradable, but are highly susceptible to being adsorbed.
The problem with such adsorbent-based systems is that they require very long residence times in the bioreactor, result in the formation of mountains of sludge which must be disposed of, and if activated carbon is used, its cost dictates that it be separated and regenerated.
A process using a high surface area, macroporous biomass support of granular activated carbon, was used to degrade phenol with immobilized cells which are lodged within the support's macropores, and on the support's surface. With the cells so immobilized, they tolerated as much as 15 gm/L of phenol in the feed. (see H. M. Erhardt and H. J. Rehm, Appl. Microbiol. Biotechnol., 21 32-6, 1985). They indicated that the carbon served as a "buffer and depot" to protect the cells, the carbon absorbing the phenol, so that the concentration of phenol in the water surrounding the bugs was low enough for them to biodegrade the phenol.
Macroporous supports used for packing in fixed beds are those supports having an open-cell pore structure, the pores being larger than 200 .mu.m, and typically are much larger, so as to offer essentially no resistance to flow of feed not only around and over the packing, but through individual pieces or pellets of the packing. Microporous supports, on the other hand, have been used for packing in fixed beds for the sole purpose of providing an anchor for the cells, the flow of feed being over and around the packing. Especially heat treated clay pellets, such as of Celite.RTM. diatomite, have been used in the past, expressly for the purpose.
Lupton et al supra, discussed several embodiments of the "adsorbent" technology and presented yet another embodiment. They used a modification of packed bed technology in a "combination" process in which a packed bed of macroporous inert material (or "packing") is combined with an adsorbent, specifically, macroporous activated carbon, which is held within the much larger pores of an open-cell foamed polyurethane. This had the advantage of entrapping both, the activated carbon within the foam, and in turn, the phenol pollutant in the activated carbon. However, the activated carbon in this system is said to concentrate pollutants on its surface so that the proximity of the microorganisms to the locally high concentration of adsorbed pollutant would result in their faster and more complete degradation. (see col 5, lines 21-28). This was precisely what we sought not to do. We wished simply to meter the pollutant to the cells at the maximum rate at which they could ingest and degrade it.
Though the Lupton et al process purported to remove essentially all phenol within a hydraulic retention time (HRT) of less than 16 hr, and to lose no carbon from the reactor, thus avoiding the need to replace the carbon, their system was designed with different parameters from those used in our invention, and as a result are ineffective to cope with highly recalcitrant feeds.